The security guarantee of information security is based on a security algorithm which is a class of asymmetric key algorithm. The basic operation of the asymmetric key algorithm is large modular multiplication, and the Montgomery modular multiplication algorithm in the modular multiplication algorithms is high in efficiency and easy to realize. An ordinary number needs to be converted into a Montgomery number and then subjected to Montgomery modular multiplication in a process of using Montgomery modular multiplication, however, it is necessary to use a conversion coefficient in the process of converting the Montgomery number, wherein the conversion coefficient is obtained by calculation generally via a large modular power multiplication algorithm, such that complex large operations are needed in the existing embedded security chip, thus needing longer operating time and more resources in the embedded security chip.